This invention relates to pressure vessels especially of cylindrical form. It is acknowledged that there is no difficulty in designing pressure vessels to vent to atmosphere in the event of exceeding the safe operating pressure; there remains the possibility, however remote, that the vessel may fracture with the result that vessel fragments become missiles which could damage adjacent structure or components. One aspect of this problem is in connection with vessels containing noxious fluids where missile-like fragments may puncture secondary containment structures provided to contain the escape of fluid.